Generally, metal products used in industry are made into shapes suitable for use through final cold work before being final products, and come to have characteristics satisfying product performance requirements through final heat treatment.
These product characteristics are directly related to a state of grains forming a microstructure, and are determined by microstructural changes caused by the final heat treatment applied after cold working.
Accordingly, the final heat treatment in terms of producing the product is the last step capable of controlling the material characteristics of the product.
Effects of the final heat treatment affecting the product characteristics may be described in detail as follows.
The cold working of a final heat treated product causes macroscopically the deformation of the grains composing the microstructure by forcing the atoms to move in a short time duration without gaining thermal assistance.
Microscopically, the generation and transfer of dislocation for deformation begins in the grains, and an increase of dislocation density due to subsequent continuous processing destroys the interatomic arrangement and eventually renders the microstructure disordered to such an extent that the grains and grain boundaries cannot be distinguished. A material composed of such a wrought structure is no longer capable of generating and transferring the dislocation, thereby having a high mechanical strength with a low percent elongation. Accordingly, improvement through heat treatment is required due to the low percent elongation.
That is, defects in the deformed grains showing embrittlement characteristics are extinguished or existing grains are replaced with newly formed defect-free grains through the heat treatment, whereby softening characteristics are added to the material.
These phenomena are called recovery and recrystallization, respectively, and generally the recrystallization occurs at relatively higher temperatures compared with the recovery. The recrystallization is expressed as a quantitative fraction called recrystallization rate which is mainly used as a heat treatment target or a material characteristic parameter capable of predicting the behavior of a material.
Eventually, the fraction of the total volume of the existing recrystallized grains can be expressed as the recrystallization rate, and this fraction is also expressed as a ratio of mechanical strength. In reality, the recrystallization rate of materials is evaluated in industry using two conventional methods as below.
A first method called recrystallization volume fraction measurement method generally uses micrographs taken by using Transmission Electron Microscopy (TEM). At this time, a fraction, SRex/Stotal of the total two dimensional area Stotal versus the recrystallized grain area SRex is defined as the recrystallization rate by using a method of a two-dimensional measurement of the three-dimensional space in the same manner as a general grain size measurement.
However, due to the nature of TEM micrographs, the maximum observable range for observing grains is so narrow in units of several tens of micrometers that information provided therein is insufficient to represent the material. In addition, the criterion for determining recrystallized grain and cold worked grain is ambiguous, whereby a result slightly different from the actual recrystallization rate value may be obtained.
A second method called mechanical strength measurement method is usually calculated using the yield strength. Specifically, the yield strength Ymin obtained by a uniaxial tensile test after completely recrystallizing the cold worked material by heat treatment is set to a 100% recrystallization rate, and the yield strength Ymax of the material in a state without heat treatment is set to a 0% recrystallization rate. Accordingly, since the yield strength Yp of the partially recrystallized material lies between 0 and 100% recrystallization rates, an amount of strength decreased from Ymax relative to a difference between Ymax and Ymin is calculated as the recrystallization rate XY,rex of the material as shown in Equation 1 below.
                                          X                          Y              ,              rex                                =                                                                      Y                  max                                -                                  Y                  p                                                                              Y                  max                                -                                  Y                  min                                                      ×            100                          ,        %                            [                  Equation          ⁢                                          ⁢          1                ]            
The yield strength is a result due to many grains, thereby being representable information of the microstructure state. However, in the case of a material heat-treated at a temperature of recrystallization temperature Trex or lower, a calculation result may be caused to be far away from the actual recrystallization rate because the reduction in strength is mainly due to the recovery rather than due to a formation of recrystallized grains.
Generally, recrystallization temperature Trex of zirconium alloy cladding of a nuclear fuel rod is about 500° C. In fact, papers dealing with the recrystallization rate modeling through the measurement of the yield strength of zirconium alloys show that the experiment has been performed excluding the cases of zirconium alloys heat-treated at 500° C. Trex or lower [Hang Tian et al., J. Nucl. Mater. 456 (2015) p. 321, Y. I. Jung et al., J. Alloys Comp. 497 (2009) p. 423].
This is attributable to inappropriate results produced from recrystallization rate modeling calculations by a reduction in strength due to the recovery rather than by a reduction in strength caused by thermally-induced recrystallization.
Since the final heat treatment temperature of zirconium alloy cladding of nuclear fuel rods used in nuclear power plants is 500° C. or less, nuclear fuel manufacturers use TEM micrographs to measure the recrystallization rate of zirconium alloy cladding. In general, a TEM micrographs is allowed to observe local areas only in the range of up to tens of square microns, whereby a number of multiple TEM micrographs are required to demonstrate the typical recrystallization rate of heat-treated specimens.
In Korean Patent No. 10-1493944, after selecting a critical value by measuring an crystal orientation spread value of a metal material by using EBSD, the recrystallization rate is measured through a process comparing the critical value with crystal orientation spread values of the crystal grains of a specific size or larger.
In U.S. Pat. No. 9,002,499, a method is proposed to determine a degree of the recovery by measuring azimuth of crystal grains of a metal material by using EBSD.
Up to now, the recrystallization rate measurement method by using EBSD, like the existing registered patents, has stated various analytical methods capable of determining whether a grain is recrystallized or not according to an azimuth difference after measuring crystallographic orientations of the grains. In addition, much literature has also reported methods for determining recrystallized grains based on crystallographic orientation.
A problem associated with these methods is that in order to determine whether a measured grain is recrystallized or not, it is necessary to set the critical crystal orientation spread value and minimum effective grain size as references through a basic experiment. In other words, when the two critical values above are set incorrectly, there is a possibility to get values different from the actual recrystallization rate of the material.
Therefore, there is a need for a technique that can identify not only occurrence of the recrystallization rather than the recovery but also a degree the recrystallization rate has progressed with a high accuracy without passing through precise measurements which entail numerous trials and errors.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.